The monitoring and control of industrial processes, for example in the chemical and pharmaceutical industries, in the textile industry, in the food and beverage industries, in the processing of paper and cellulose, or in the fields of water processing and waste water treatment, is based on the measurement of process variables that are determined by means of suitable measuring probes.
According to “Process Measurement Solutions Catalog 2005/June”, Mettler-Toledo GmbH, CH-8902 Urdorf, Switzerland, pages 8 and 9, a complete measuring system consists of a housing, a measuring probe, a cable and a measurement converter (also called a transmitter). By means of the housing, the measuring probe is brought into contact with the process that is to be measured or monitored, for example by immersing the probe in the process material and holding it there. The measuring probe serves to measure specific properties of the process. Measurement signals are sent through the cable to the transmitter, which communicates with a process control system and converts the measuring signals into readable data. The measuring probes are selected depending on the process material properties that are to be measured.
Typically, an electrochemical measuring probe such as for example a pH-measuring probe or an oxygen-measuring probe is subject to a load-dependent wear process which is inherent in the functional principle of the probe and which normally leads to a continuous change of the measurement characteristics of the measuring probe.
In order to take into consideration the impact of such changes into the accuracy of measurements, it is known from the state if the art to monitor essentials electrical characteristics of the signal measuring circuit of a probe, such as the resistivity. In particular, it is known to feed the signal measuring circuit of a probe with a test signal generated by a stimulation circuit, as disclosed for example in patent document US 2009/0251152 A1. For example, for determining the resistance of the signal measuring circuit connected to the pH glass of an electrode, a rectangular impulse signal is switched onto the measuring signal, the resulting combined signal being then passed through electronic filters and like correctors to determine said glass resistance. Condensers, operated in parallel to the measuring signal, come generally in between the stimulation circuit and the signal measuring circuit, to dynamically shape the test signal.
However, in this configuration, leakage currents between the signal measuring circuit and the electrically connected stimulation circuit, cannot be avoided, and may affect accuracy of measurements adversely. This issue is even more relevant for measuring probe delivering weak measuring signal, for example pH measuring probe provided with an electrode with a high pH-glass resistance, typically greater than 1 Gohm.
To mitigate these problems, patent document GB 2 333 161 A discloses for example means for limiting leakage currents from the signal measuring circuit to the stimulation circuit. However the measuring signal is still influenced by the stimulation circuit, since the signal measuring circuit is coupled and influenced by the glass resistance circuit used for testing/diagnostic reasons. Moreover, the described solution does not allow the direct determination of the impedance of the glass electrode on its own, but instead of the circuit formed by the glass electrode immersed in the process material.
That's why there is still a need for improved means for determining, during a verification phase, in a measuring system comprising a measuring probe for use in contact with a process material, at least one electrical characteristic of a sensing element of an electrode included in the measuring probe, adapted to be used with measuring probes delivering weak measurement signal, connected to the measuring signal circuit.